Treatment of acid mine water waste



Unite States Patent 3,388,658 TREATMENT 6F ACID MINE WATER WASTE LouisF. Wirth, In, Western Springs, 111., assignor to Nalco Chemical Company,(Chicago, Ill., a corporation of Delaware No Drawing. Filed Aug. 3,1966, Ser. No. 569,840 9 Claims. (Cl. Mil-32) When coal deposits areexposed to natural weathering and erosion, the iron sulfides containedin the coal and adjacent strata oxidize to form new compounds, primarilyferrous sulfate and free sulfuric acid. The ferrous sulfate is thenoxidized to ferric sulfate by means of oxygen in the atmosphere. In anexcess of water, the ferric sulfate hydrolyzes to insoluble ironhydrates known as yellow boy and sulfuric acid. The above is acceleratedby removal of the coal bed through mining whereby large surface areasare exposed to oxidation. Mining may also drain ground water from thesurrounding strata and this seepage will dissolve the acid salts formedin the mine and transport them to the streams. The acid mine drainageproblem is particularly present where there is extensive coal mining, anabundance of rainfall which produces large quantities of rainwater andrun-off, a low natural alkalinity of streams and presence of pyrite incoal.

With more specific regard to this problem, coal associated with irondisufilde usually is isolated from oxygen and water from its naturalenvironment in the earth. However, mining the coal seam removes supportfrom the overlying strata, inducing cave-in. Water and air influxtogether with exposure of traditional acid-producing materials, co-actto produce the acidic Waste waters. The problem is particularly severewhen mining activity is ceased. In these abandoned mines, watercontinues to flow indefinitely. For example, in one small abandonedunderground mine, it was found, via random sampling, that the total acidload, in terms of the mine discharge over a period of 182 days, wasequal to 41 tons of sulfuric acid. The acid load for the entire yearwould equal approximately 80 tons, which acidity flowed directly into amajor river.

The reactions which occur in formation of acid mine waters are notexactly known. However, the following reactions have been generallyaccepted as typical of the chemical change occurring in the coal seamitself and surrounding rock strata in presence of air and water.

Ferric sulfate Water Ferric hydrate Sulfuric Acid t ansom +6H O 2Fe(OH)3113804 Additional ions such as silica, aluminum, manganese, cal

"ice

cium and magnesium are also present in a typical acid mine water, andthese ions are thought to accelerate formation of acid. The abovereactions generally occur when ground water pcrcolates downward throughthe overburden of the mine, passes through the mining workings and thendrains into streams or rivers.

As is readily apparent, the acid mine drainage causes a severe problemof pollution and particularly causes extensive fish kill. The drainagefrom mine sites usually reaches the larger rivers via small tributaries.In many states, the acidic mine drainage waters have been classified asan industrial waste and disposal of such waters into streams clean orpolluted has been prohibited. Millions of gallons of mine watercontaining sulfuric acid, ferrous sulfate and ferrous hydroxide aredischarged annually into bodies of water in varying amounts of acidity uto about 5,000 ppm. It is therefore evident that the problem is one ofsome magnitude.

A number of proposals have been made in an effort to overcome thisproblem. For example, it has been advocated that suflicient coal bepermitted to remain in place to prevent cave-in. However, there is asevere eco nomic loss in coal left behind. Also in many coal seams,systematic pulling of pillars and controlled caving yield the onlypractical method of relieving excessive rock pressures. The excavations,of course, materially contribute to the problem of production of acidmine waters.

Another proposal made involves sealing of abandoned mines to exclude airand prevent oxidation of sulfide material. However, the highinfiltration rate and permeability of the over-burden to water has madethis suggestion a poor solution to the problem at hand.

It would be of extreme advantage to the art if a method were discoveredof somehow treating acid mine discharge waters whereby particularly theiron and acidity were removed therefrom prior to discharge of thesewaste waters into streams or rivers. If such acid mine discharge waterscould be somehow purified prior to drainage into major tributaries, theproblem of water-pollution of many bodies of water could be overcome toa substantial degree.

Therefore, it becomes an object of the invention to purify acidic minedischarge waters containing iron and sulfuric acid impurities.

A specific object of the invention is to purify the above mine watersvia a unique ion exchange system and thereby avoid pollution of bodiesof water into which said mine waters generally feed.

In accordance with the invention, a method of purifying acidic minedischarge waters has been discovered. These waters which contain asimpurities at least iron and sulfuric acid have been purified via themethod of the invention by contact with ion exchange resins. Moreparticularly, the discharge waters are purified by bringing them incontact with an anion exchange resin in the sulfate form, whereby thesulfuric acid content is reduced. The waters are also contacted with acation resin having on its exchange sites cations selected from amongcalcium, magnesium, sodium and potassium which are capable of beingreplaced by iron ions whereby the iron content in the waters is reduced.

The contact of water with resins may be carried out in any sequence.That is, the water may be first contacted with the anion resin followedby treatment with cation resins. The reverse procedure may also becarried out. In yet another embodiment, a mixed bed of cation and anionresins may be utilized. The method of the invention is particularlyadaptable to treating acidic mine discharge waters which have an ironcontent ranging from about 5 to about 5,000 ppm, expressed asFe and afree mineral acid content of 15,000 p.p.m., expressed as CaCO In apreferred embodiment, the free mineral acidity, chiefly sulfuric acid,is removed by a first contact with the anion resin in sulfate form. Thetype of the anion resin may vary widely, and the resin itself may beeither a strongly basic or weakly basic liquid or solid material. Theprimary requisite is that the anion exchanger be in sulfate form.

Preferred anion exchange resins used in the practice of the inventionare strongly basic anion exchange resins, i.e. anion exchange resinswhich in the hydroxide form are capable of converting inorganic salts inaqueous solution directly to hydroxides. Thus, a strongly basic anionexchange resin is capable of converting an aqueous solution of sodiumchloride directly to an aqueous solution of sodium hydroxide. A stronglybasic anion exchange resin can also be defined as one which on titrationwith hydrochloric acid in water free from electrolytes has a pH above7.0 when the amount of hydrochloric acid added is one-half of thatrequired to reach the inflection point (equivalence point). A weeklybasic anion exchange resin under the same conditions has a pH below 7.0when one-half of the acid required to reach the equivalence point hasbeen added. The strongly basic anion exchange resins which are availablecommercially are characterized by the fact that the exchangeable anionis a part of a quaternary ammonium group. The quaternary ammonium grouphas the general structure:

| Rs -NR;

wherein R R and R represent alkyl or substituted alkyl groups, and X" isa monovalent anion.

Examples of the strongly basic anion exchange resins which can beemployed in the practice of the invention are those resins disclosed inUS. Patents 2,591,573, 2,597,440, 2,597,494, 2,614,099, 2,630,427,2,632,000 and 2,632,001.

Strongly basic insoluble anion exchange resins include reaction productsof a tertiary alkyl amine and a vinyl aromatic resin having halo methylgroups attached to aromatic nuclei in the resin which resins aresubsequently converted to the sulfite form. Another class of stronglybasic anion exchange resins suitable for the practice of the inventionare the reaction products of tertiary carbocyclic or heterocyclic aminesand vinyl aromatic resins having halo methyl groups attached to aromaticnuclei in the resin which resins are subsequently converted to thesulfite form.

The vinyl aromatic resins employed as starting materials in making theanion exchange resins employed in the preferred practice of theinvention are the normally solid benzene-insoluble copolymers of amonovinyl aromatic compound and a polyvinyl aromatic compound containingfrom 0.5 to 40% by weight, preferably from 0.5 to 20% by weight of thepolyvinyl aromatic compound, chemicallycombined with 99.5% to 60% byWeight of the monovinyl aromatic compound. Examples of suitablemonovinyl aromatic compounds are styrene, alpha methyl styrene,chlorostyrenes, vinyl toluene, vinyl naphthalene, and homologuesthereof, capable of polymerizing as disclosed, for example, in US.Patent 2,614,099. Examples of suitable polyvinyl aromatic compounds aredivinyl benzene, divinyl toluene, divinyl xylene, divinyl naphthaleneand divinyl ethyl benzene. These resins are halo methylated asdescribed, for instance, in US. Patent 2,614,099, preferably tointroduce an average of 0.2 to 1.5 halo methyl groups per aromaticnucleus in the copolymer and then reacted with a tertiary amine tointroduce a quaternary ammonium anion exchange group. Examples ofsuitable tertiary amines are trimethyl amine, triethyl amine, tributylamine, dimethyl propanol amine, dimethyl ethanol amine, methyldiethanolamine, 1-methylamino-2,3-propane diol, dioctyl ethanolamine,and homologues thereof.

The anion exchange resins can also be prepared by halogenating themolecule of the resin and then introducing an anion exchange group asdescribed in U.S. Patent 2,632,000 and subsequently converting them tothe sulfite form, with or without admixture with the hydroxide form ofthe resin.

Specific anion exchange resins that can be used as starting materials inpracticing the invention include Dowex SAR and Dowex SBR. Dowex SBR is astyrenedivinylbenzene resin containing quaternary amine ion exchangegroups in which the three R groups are methyl groups. This resinconsists of spherical particles of 20 to 50 mesh and containing about40% water. The divinylbenzene content is approximately 7.5%. The totalexchange capacity is approximately 1.2 equivalents per liter, wetvolume. Dowex SAR is similar to Dowex SBR except that one of the methylgroups in the quaternary amine salt structure is replaced by a hydroxyethyl group. Dowex SBR is somewhat more basic than Dowex SA R.

From a regeneration or conversion standpoint, the HSO =:SO reaction isindependent of the anion resin used. For this reason both strong baseanion exchange resins and weak base anion exchange resins arecontemplated within the scope of this invention. The commerciallyavailable products Dowex WGR and Dowex WBR are examples ofpolyamine-type weak base resins. Such resins usually contain a mixtureof primary, secondary, and tertiary amine groups.

The cation exchange resins are employed to remove or reduce the ironcontent in the waste water. In one preferred embodiment, the cationresin is utilized subsequent to employment of the anion exchange resindescribed above. The cation resin should have on its exchange sites acation selected from the group consisting of calcium, magnesium,potassium and sodium. Of these, it is most preferred that the cationresin be in calcium form. Use of such. calcium form resin in an ionexchange process is believed to be unique, since the usualwater-softening procedure involves the opposite from what is to becarried out here. That is, generally hardness such as calcium is removedfrom water via contact with cation exchange resin in alkali metal formsuch as sodium. In the instant invention, just the contrary is takingplace; that is, the calcium is being exchanged for iron and hardness isbeing introduced into the mine water. However, introduction ofhardness-containing constituents by calcium exchange into the treatedwater is not a draw back in the instant inveniton, since the purifiedwater, when finally fed into a river or stream will be subsequentlysoftened if utilized for consumer use. Thus, it is believed that this isthe first use of the calcium form cation exchange resin as a means ofpurification of water.

The cation exchange resins themselves are known in the prior art. One ofthe most common types is a sulfonated resin. Nalcite HCR-W is a typicalresin of this type, and is a sulfonated styrene divinyl benzene stronglyacid cation exchanger of the type described in US. 2,3 66,- 007. Thus,all that is necessary in order to utilize this resin is to put it in acalcium or other suitable form.

Another suitable form of cation exchange resin is a sulfonate acidphenol-formaldehyde resin, such as a resin derived by condensing aphcnolsulfonic acid with formaldehyde. In general, resins having aplurality of sulfonic acid groups are the most suitable cation exchangeresins suitable for this invention. Synthetic zeolites may also be usedin this step.

After a certain period of use, the anion resin becomes exhausted andregeneration is necessary. Any method of conventional regeneration ofanion resins may be utilized, as long as the bisulfate form of theexhausted resin is converted to the useful sulfate form. One preferredmethod is to rinse the exhausted anion bed with Water. The Water can beflowed either downwardly or upwardly through the anion exchange resin.In a particularly preferred embodiment, the purified acid mine water isutilized as an anion exchanger regenerant, thus making the overallprocess more attractive economically.

The conversion of anion exchange resin bisulfate to the sulfate form canbe accelerated by using an alkaline rinse Water, whether the source ofwater be the purified acid mine water or another source of rinse Water.A greatly preferred alkaline compound used as the make-up for thealkaline rinse water is lime, preferably in slurry form. Other alkalinecompounds may also be used such as, for example, ammonium hydroxide,sodium bicar bonate, sodium hydroxide, potassium bicarbonate andpotassium hydroxide. The use of an alkaline compound accelerates theregeneration step, although untreated water may also be usefullyemployed.

Again, when the cation resin becomes exhausted, a regeneration step isalso necessary. One mode of cation regeneration is treatment of thecation exchange resin with an aqueous sulfuric acid whereby a cationhydrogen form resin is realized. This resin is then put in an alkalimetal or alkaline earth metal form. However, the most eflicient way ofcarrying out the cation resin regeneration is by contact of the resinhaving a majority of its sites attached to iron ions with an alkalineearth or alkali metal salt or hydroxide. The most preferred procedure isto treat the exhausted cation exchanger with a source of calcium ionssuch as by treatment with calcium hydroxide or calcium chloride. Themost efiicient mode of regeneration is contact of the cation exchangeresin with an aqueous solution of a calcium salt such as calciumchloride.

A greatly preferred system of purifying acid mine waters includingregeneration of resins employed is as follows: A series of four reactorsare provided. These contain anion resin, regenerant for anion resin,cation resin and regenerant for cation resin.

The regenerant reactor for the anion resin is first filled with a sourceof neutral Water which can be periodically replenished with etfiuentfrom the service run of the cation exchange resin. To this water is thenadded lime, preferably in slurry form consisting of dissolved lime andlime suspended in aqueous medium. The lime slurry preferably has aconcentration range of about 2,000 to 10,000 p.p.m., and most preferably2,500 to 4,000 p.p.m., total alkalinity as CaCO After the anion exchangeresin is exhausted, approximately /2 of the lime to be utilized asregenerant is added to the neutral water and this regenerant solution ispassed up-flow through the exhausted anion exchange resin. The efiiuentfrom the regenerant run is returned to the regenerant solution and theremainder of the lime slurry added to regenerant solution. Theregenerant solution thereby becomes saturated with calcium sulfate whichprecipitates out of solution and is drawn off in some manner such as inform of a filter cake or sludge or slurry cake from a centrifugationoperation. The calcium sulfate cake is then disposed of in some manner.The filtrate or concentrate is, of course, available for subsequentregenerations.

Again, the regenerant used for the cation exchange resin may be a sourceof neutral water. To this is added calcium chloride, and this calciumchloride solution is also flowed upward through the exhausted cationresin. The regeneration effluent is again added back to the reactorcontaining regenerant solution. To replenish the source of calcium, limeis added to the cation regenerant solution. This causes the iron nowpresent to precipitate as an iron hydroxide. Generally, a source ofoxygen is also added to this regenerant solution in order to convert theiron from ferrous to ferric form, thereby causing precipitation of theiron. The ferric hydroxide may then be used as a suitable source of ironin a steel making operation.

When additional water is needed to replenish the regenerant solutions,the purified acid mine water may be utilized. Thus, once the process hasbeen set up, it may be run in a continuous manner without need to resortto fresh regenerant solutions upon each regeneration of anion and cationresin. As is evident, the process then is one which is extremelyattractive both from technical and economic viewpoints.

The following example relates to a typical procedure of the invention inpurifying acid mine water waste. It is understood, of course, that thisexample is merely illustrative and that the invention is not to belimited thereto.

EXAMPLE I A synthetic test water was prepared to simulate thecomposition of a typical acid mine water. Analysis of this acidic wastewater is as follows:

Free mineral acid (FMA) 880 p.p.m. as CaCO Iron (total Fe+ +Fe+ 516p.p.m. as Fe. Iron, Fe+ 11 ppm. as Fe. Calcium hardness 980 p.p.m. asCaCO Sulfates, total 1920 p.p.m. as 50 Chloride 15 p.p.m. as NaCl.Aluminum 24 p.p.m. as Al. Manganese l6 p.p.m. as Mn. Sodium 17 p.p.m. asCaCO Magnesium 34 p.p.m. as CaCO pH 2.15.

A 3 inch inner diameter tube was filled with 4 liters (0.141 cu./ft.) ofa strong base anion resin (Dowex SBR) in sulfate form. The resin bedheight was 35". The acid mine water was then flowed through the anionresins until exhausted. Random sampling of the efiiuent, throughout thebulk of the run, indicated that the free mineral acidity was reduced toessentially 0 p.p.m. The service run was carried out by passage of theacidic water downflow through the anion resin at a flow rate of 0.9g.p.m. per cu. ft. resin at a temperature of 78- 80" F. Under theseexperimental conditions and operating with an acid water of 880 p.p.m.of free mineral acidity, a throughput capacity of 8085 gallons of minewater per cu. ft. of resin was obtained. The run was terminated andresin regenerated when the effluent reached an acidity of 200-250 p.p.m.free mineral acid, expressed as CaCO The efiiuent from the anionexchange resin was then passed through a 1500 ml. resin column of astrong acid cation exchanger completely in the Ca++ form at a flow rateof 2.4 g.p.m. per cu. ft. of cation resin (Dowex HCR-W). The capacityfor iron was 0.814 pound of iron, expressed as Fe, per cu. ft. of cationresin.

When exhausted, the anion exchange resin was regenerated and put back insulfate form by treatment with lime water. More specifically, theregeneration was carried out by first purifying the resin with 3.5 to 4gallons of deionized water per cu. ft. resin to remove acid mine waterentrapped in the resin column. Lime water containing 2250 p.p.m. ofcalcium hydroxide, expressed as calcium carbonate, was then passedupfiow through the resin to obtain a 20% expansion or at a flow rate of0.85 g.p.m. per cu. ft. of resin. Any excess lime water left in thecolumn was displaced by downflow wash with 3.5 to 4 gallons of deionizedwaters before starting a new exhaustion run of acid mine water.

The iron exhausted cation resin was regenerated in the following manner.The resin was first purified with deionized water to a 50% expansionuntil all precipitated iron was removed. This iron was collected andanalyzed: 0.017 pound of Fe was found. The resin was then regeneratedwith 50 pounds of calcium chloride per cu. ft. of resin, applied as a10% solution of calcium chloride. This solution was passed through theresin at 0.6 g.p.m. per cu. ft. This arnount of calcium chloride eluted0.687 pound of iron per cu. ft. resin or 84.4% of the iron on the resin.The elution of total amount of iron on the resin was then contemplatedby contact of the resin with 3 normal hydrochloric acid solution at F.until no more iron was found in the efiluent. The amount of iron removedin this step was 0.11 pound per cu. ft. resin.

In addition to removing iron, the process of the invention also removesother contaminant metal ions from acid mine water waste such asmanganese and aluminum.

The invention is hereby claimed as follows:

1. A method of purifying acidic mine discharge water which contains asimpurities at least iron and sulfuric acid which comprises bringing saidwater into contact with a strong base anion exchange resin in thesulfate form to reduce said sulfuric acid content, and with a cationresin having on its exchange sites cations selected from the groupconsisting of calcium, magnesium, potassium and sodium which are capableof being replaced by iron ions whereby said iron content in said wateris reduced.

2. The method of claim 1 wherein the iron content in said mine Waterranges from about 5 to about 5000 p.p.m., expressed as Fe, and saidwaters, also contain 1-5000 p.p.m. of free mineral acid, expressed asCaCO 3. The method of claim 1 wherein said contact is effectedsequentially by treatment with said anion resin followed by treatmentwith said cation resin.

4. The method of claim 3 wherein the iron content in said mine waterranges from about 5 to 5000 p.p.m., expressed as Fe, and said watersalso contain 1-5000 ppm. of free mineral acid, expressed as CaCO 5. Themethod of claim 1 wherein said anion resin and said cation resin are inmixed bed form.

6. The method of claim 1 wherein said anion exchange resin is selectedfrom the group consisting of a strongly basic solid anion exchanger, aweakly basic solid anion exchanger, a strongly basic liquid anionexchanger and a. weakly basic liquid anion exchanger.

7. A method of purifying acidic mine discharge Water which contains asimpurities at least iron and sulfuric acid which comprises bringing saidwater into contact with an anion exchange resin in the sulfate form toreduce said sulfuric acid content, and with a cation resin in calciumform whereby said iron content in said water is reduced, regeneratingsaid anion resin by contact with water, and regenerating said cationresin by contact with a water-soluble calcium salt.

8. The method of claim 7 wherein said water regenerant is an aqueousdilute base and said calcium salt is calcium chloride.

9. The method of claim 8 wherein said basic regenerant is selected fromthe group consisting of lime solution and lime slurry.

References Cited UNITED STATES PATENTS 2,628,165 2/1953 Bliss 210-38 X2,660,558 11/1953 Juda 21032 X 2,738,322 3/1956 Baumanet al. 210-322,954,276 9/1960 Hazen 2l.O38 X SAMIH N. ZAHARNA, Primary Examiner.

7. A METHOD OF PURIFYING ACIDIC MINE DISCHARGE WATER WHICH CONTAINS ASIMPURITIES AT LEAST IRON AND SULFURIC ACID WHICH COMPRISES BRINGING SAIDWATER INTO CONTACT WITH AN ANION EXCHANGE RESIN IN THE SULFATE FORM TOREDUCE SAID SULFURIC ACID CONTENT, AND WITH A CATION RESIN IN CALCIUMFORM WHEREBY SAID IRON CONTENT IN SAID WATER IS REDUCED, REGENERATINGSAID ANION RESIN BY CONTACT WITH WATER, AND REGENERATING SAID CATIONRESIN BY CONTACT WITH A WATERSOLUBLE CALCIUM SALT.